Grain fumigation  a new look

April 01, 1998
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by Teresa Acklin

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Australian researchers develop different delivery systems for phosphine application.

By Chris Newman

Fumigation remains one of the most important weapons in the armory of grain storage managers in their battle against insect infestations. Chemical pesticides are fast becoming redundant in many countries because of their high cost, the development of insect resistance to them and the increasing disaffection of consumers who do not want poisons mixed with their foodstuffs.

Aeration remains a useful tool for controlling insects, but it cannot be relied upon to kill them. Controlled atmospheres of carbon dioxide (CO2) and nitrogen now provide a means of supplying the market with “organic” grain that is free from all chemical intrusion, but they are mostly too expensive for general use and are only able to be used in extremely well-sealed storage. The importance of fumigation can therefore be expected to increase in the world's marketplace where insect-free grain is becoming the accepted standard for export and trading purposes.

Only two fumigants, methyl bromide and phosphine (PH3), remain in common use in the grain industry, and of these, the future of methyl bromide is expected to be short-lived because of its harmful effects on the atmosphere. While alternatives such as carbonyl sulphide are being studied by researchers in Australia, it can be expected that it will take many years before they are registered for use or manufactured in commercial quantities. The world will therefore increase its reliance on phosphine as the main fumigant for control of insects in its grain stocks.

Phosphine Basics.

Despite the widespread use of phosphine throughout the world, there is surprisingly little understanding within the grain industry of how it should be used to control insects. Research conducted by Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO), the country's leading research agency, and others over the past 20 years has conclusively demonstrated that not only does phosphine have to be applied at low concentrations for long periods of time to ensure insect mortality, but also that insects can develop resistance to it if it is not administered properly.

Ineffective fumigations will select resistant insects through the survival of the “fittest.” The survivors of repeated fumigations will, over a period of time, produce resistant strains of insect, which become increasingly difficult to kill.

Insects develop over a four-stage life-cycle. Eggs hatch into larvae, which become immobile pupae before finally developing into adults. The eggs and pupae are “immobile” phases; only the larvae and adults provide visual evidence of an infestation.

The mobile larvae and adults are easy to kill with phosphine, which is why the belief remains so common that a “quick” fumigation will do the job. However, the immobile phases are much more difficult to kill, and if allowed to survive, infestation will become visible again within a month or two after a fumigation (see Diagram 1). Effective fumigation must aim at killing all the developmental stages of the insects.

Effective fumigation can only be achieved by administering low doses of phosphine over long periods of time and by ensuring that a minimum dosage is distributed and maintained throughout the grain mass over the full fumigation period. Some adult insect species take several days before they begin to absorb phosphine and will survive short fumigations even at high concentrations.

Excessively high dosages can cause insects to become narcotized to the extent that they become comatose and absorb no phosphine at all. By applying low dosage rates over long periods (15 to 28 days), all eggs and pupae are given time to develop into larvae and adults, which are most easily killed. Application rates and minimum concentration requirements are temperature dependent; recommended figures for a range of fumigation periods can be seen in the table above (for tolerant species of insect).

Sealed Storage.

Phosphine is normally generated by applying aluminium or magnesium phosphide tablets, either by mixing them with the grain or placing them inside the grain storage enclosure. The gas is released through chemical reaction with the moisture in the air. The reaction time may take one to three days or more, depending on the grain moisture content and temperature.

Sufficient phosphine must be applied to achieve the required minimum dosage rate at the end of the fumigation period, with the result that a high initial dose has to be applied in order to allow for losses that will occur through sorption into the grain and leakage from the storage. A lot of the fumigant is lost and effectively wasted through these effects. If loss of fumigant is so great that the concentration of gas is too low at the end of the fumigation period, then the fumigation will be ineffective.

The only way to ensure that enough gas is retained in the grain during a fumigation period is to seal the storage to an acceptable degree of gas-tightness. No storage can be completely gas-tight, and some loss of gas is inevitable. The degree of gas-tightness is best determined by “pressure-testing” the storage, which is done by applying a positive or negative air-pressure to it and measuring the time it takes for the pressure to drop to half its starting value.

The starting pressure should suit the type of storage so that it does not cause structural damage (in the order of 0.5 kPa or less for horizontal steel storages and perhaps 1.0 to 1.5 kPa for concrete silos). An acceptable pressure test is one where the half-life pressure decay is in excess of 8 minutes if the storage is full or 15 minutes if it is empty.

In Australia, storages of every size and type have been sealed to achieve this target, but some are easier and less costly to seal than others. Fumigating with tablets in storages that do not meet this standard is likely to be ineffective.

Of particular importance is sealing the tops and bottoms of vertical silos, since temperature differentials between the grain and the outside air can cause massive air-movements through any top and bottom openings by what is called the “chimney effect.” It has been demonstrated that if cold air leaks into the bottom of a silo full of warm grain and escapes through the roof (or if warm air leaks into the roof of a silo full of cold grain and escapes through the bottom), a complete air change within the silo can take place within a matter of one or two days or hours in some instances, which can quickly remove all traces of phosphine from the silo.

Tablet Application Methods.

Phosphide tablets are normally applied by placing them inside the storage, either by admixing them with the grain or by placing them in trays or in “blankets” above the grain mass.

Admixture: Mixing the tablets with the grain requires the grain to be moved so that the tablets can be dropped onto the grain as it is carried on a conveyor or flows down a chute. This method imposes many disadvantages:

 it requires infested grain to be moved through the plant, spreading the infestation through the plant;

 generation of phosphine may begin to take place while the grain is on a conveying system, with consequential risks to worker safety especially if the conveying system stops for any reason;

 a lot of phosphine is released during the time the storage is being filled and will be lost through the displacement of air from the silo; some of it may find its way into working areas;

 it is not possible to control the concentration of phosphine: if the concentration is found to be too low (because of losses through grain sorption or air leaks), it is difficult or impossible to add more tablets to increase the concentration;

 there is a risk to operators if the grain is removed from the silo before all the phosphine is released and ventilated from the storage;

 phosphine residues will remain in the grain: up to 5% of unreacted phosphide may be detectable. Deaths from phosphide poisoning have been recorded where residue-contaminated grain has been fed to animals. In addition, aluminium (or magnesium) oxide residues will be left in the grain.

For all these reasons, fumigation by the addition of phosphine to the grain is no longer a recommended practice in Australia.

Placement: Placement of tablets into the storage is a preferred method of fumigation, since it is easier to control the fumigation (more tablets can be added if required) and no residues are left in the grain. In flat storages or “squat” silos, in which the height to width ratio is less than 1.5, natural air convection will distribute the gas through the grain mass and there is no need for forced air recirculation. In tall vertical silos, a small fan and recirculation ducting are needed to recirculate the gas and to ensure that the gas is distributed throughout the grain mass.

Tablets may be spread out in a thin layer on aluminium trays that are placed in the head-space of the storage. Alternatively, “sachet” formulations may be used where the tablets are supplied in fabric sachets, which, when joined together, form “blankets” that can be easily spread over the surface of the grain.

Care must be taken to prevent the formation of a high concentration of phosphine around or within the tablets, since this may cause spontaneous combustion of the gas. The tablets must be spread out in a thin layer with plenty of space for air movement around them.

In all cases, the storage must be gas-tight to ensure that the phosphine is retained long enough to kill all stages of insects in the grain mass.

New Approaches.

New methods of phosphine fumigation have been developed in Australia over the past 10 years that overcome all of the problems associated with tablet fumigation. These methods have been developed by Bob Winks of the Stored Grain Research Laboratory of CSIRO and are marketed under the trade names of SIROFLO™ and SIROCIRC™.

SIROFLO™ and its derivative SIROCIRC™ have been specifically developed to allow leaky silos to be successfully fumigated by maintaining sufficient concentration of phosphine throughout an entire grain mass for a long enough time to kill all stages of all insect species.

Both systems offer these features:

 complete control of the fumigation process by allowing the fumigator to adjust the gas concentration and the fumigation time;

 correct and even distribution of phosphine throughout the grain mass;

 reduced risks because only very low gas concentrations are used and the phosphine is supplied in a non-flammable form;

 reduced amounts of phosphine are absorbed by the grain because the concentrations are so low; and

 no residues (such as aluminium oxide or unreacted aluminium phosphide) are left in the grain mass.

The new systems achieve these results by applying phosphine in a gaseous, rather than a solid, formulation. The gas formulation was developed by BOC Australia and marketed under the brand name Phosfume®. It is now being marketed worldwide by the BOC Group (previously known as the British Oxygen Corporation) under the name ECO2FUME®.

The CO2 acts as a dilutant to keep the phosphine below its flammability limit, hence there is no risk of the phosphine catching fire or exploding. The amount of CO2 in the mixture has no effect on the grain or on live insects, and once released into a storage it becomes undetectable against the much larger “background” concentration of CO2 that is naturally generated by the grain itself.

The system uses the gaseous phosphine mixture by controlling the discharge rate of gas from the cylinder (or cylinders) such that it is released slowly and continuously into the grain mass over the entire fumigation period. The gas is fed into a duct where it is mixed with air, and the air-gas mixture is blown at low pressure through the ducting by means of a small fan into the bottom of the grain mass. Ducting is usually small-diameter PVC, which is easy and quick to install. Diffusion of the gas into the bottom of the grain mass can be done through aeration ducting, or if this is not fitted, a low-cost “diffuser,” often in the form of an open-ended duct, can easily be designed and fitted to meet the requirements.

The fan is sized to maintain a slight positive air pressure within the storage such that the air/gas mixture leaks out through any holes in the silo structure; this prevents any air from leaking in and diluting the gas concentration. The gas and air flows are separately controlled so as to maintain the required minimum concentration of phosphine throughout the storage for any desired period.

The system is so flexible that any number of silos and any combination of sizes and shapes of silo can be fumigated at the same time (see Diagram 2). Control of the system is very simple and very safe, and training requirements are minimal. Typically, this type of fumigation maintains a minimum phosphine concentration of 20 ppm in a storage for a 28 day period, or where shorter fumigations are required, a 35 ppm minimum concentration is maintained for 15 days.

SIROCIRC™ is a recent development of the earlier system and differs from it in that phosphine is collected from the head-space above the grain mass, recirculated back through the fan and pumped back into the storage. This recirculating system can achieve a significant reduction in gas consumption depending on the “leakiness” of the storages and the amount of gas that is lost from them.

An automatic control system monitors the concentration of gas that is delivered to the storages and adjusts the flow of gas from the cylinders to maintain the required inlet concentration at the base of each storage. Re-use of the gas in this system may reach 90% in well-sealed storages.

A third method of fumigating with phosphine gas, called SIROFUME™, is a direct periodic treatment and involves adding gas from cylinders to maintain gas concentration in a storage that is left sealed for the fumigation period. Like tablet fumigations, it can only be properly carried out in an effectively sealed storage, but its major advantages are that maximum gas concentration is achieved instantaneously, reducing the overall time required to fumigate; extra fumigant can be added safely and at any time to top-up the concentration; and there are no residues left in the grain.

The CSIRO flow technology is well accepted in Australia, where it is widely used throughout the grain industry. The circulating system has only recently been introduced to the market, and only three systems have so far been installed in Australia. The fourth and most recent circulating system project has just been installed in Beijing as an introductory demonstration of the technology to China. The Beijing Central Grain Depot provided the site for the demonstration project.